Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
For the purposes of this discussion, an LED can be viewed as having three layers, the active layer sandwiched between two other layers. The active layer emits light when holes and electrons from the outer layers combine in the active layer. The holes and electrons are generated by passing a current through the LED. In one common configuration, the LED is powered through an electrode that overlies the top layer and a contact that provides an electrical connection to the bottom layer.
The cost of LEDs and the power conversion efficiency are important factors in determining the rate at which this new technology will replace conventional light sources and be utilized in high power applications. The conversion efficiency of an LED is defined to be the ratio of optical power emitted by the LED in the desired region of the optical spectrum to the electrical power dissipated by the light source. The electrical power that is dissipated depends on the conversion efficiency of the LEDs and the power lost by the circuitry that converts AC power to a DC source that can be used to directly power the LED dies. Electrical power that is not converted to light that leaves the LED is converted to heat that raises the temperature of the LED. Heat dissipation often places a limit on the power level at which an LED operates. In addition, the conversion efficiency of the LED decreases with increasing current; hence, while increasing the light output of an LED by increasing the current increases the total light output, the electrical conversion efficiency is decreased by this strategy. In addition, the lifetime of the LED is also decreased by operation at high currents. Finally, resistive losses in the conductors that route the current to the light emitting area and in the highly resistive p-layer of the LED increase rapidly with increasing current. Hence, there is an optimum current.
The driving voltage of an LED is set by the materials used to make the LED and is typically of the order of 3 volts for GaN-based LEDs. A typical light source requires multiple LEDs, as a single LED running at the optimum current does not generate enough light for many applications. The LEDs can be connected in parallel, series, or a combination of both. If the LEDs are connected in parallel, the driving voltage is low, typically of the order of 3 volts, and the current requirements are high. Hence, series connections are preferred to avoid the power losses inherent in such high current arrangements. In addition, converting the AC power source available in most applications to the DC source needed to drive the LEDs is significantly cheaper if the output driving voltage of the power supply is closer to the AC source amplitude. Accordingly, arrangements in which the LEDs are connected in series to provide a higher driving voltage for the array are preferred.
The series connections are either provided by wiring that connects the individual LEDs in the light source or by fabricating the LEDs in an array on an insulating substrate and electrically isolating each LED from the surrounding LEDs. Serial connection electrodes in this later case are then provided between the isolated LEDs by utilizing photolithographic methods. While the second method has the potential of providing reduced packaging costs, it is limited to fabrication systems in which the LED layers are grown on an insulating substrate such as sapphire so that the individual LEDs can be isolated by providing an insulating barrier such as a trench that extends down to the substrate between the individual LEDs.
There are significant cost advantages associated with fabricating LEDs on certain non-insulating substrates such as silicon wafers. Conventional fabrication lines are optimized for silicon wafers. In addition, silicon wafers are significantly cheaper than sapphire wafers. Finally, the process of singulating the individual light sources from a silicon wafer is substantially easier than the corresponding singulation process in a sapphire-based system. Accordingly, a method for generating series connected LEDs on silicon wafers or other conducting substrates is needed.